As a method replacing a conventional radiography using a combination of a radiographic film and radiographic intensifying screen, a radiation image recording and reproducing method utilizing a stimulable phosphor is practically employed. This method employs a stimulable phosphor sheet (i.e., radiation image storage panel) comprising a stimulable phosphor, and comprises the steps of causing the stimulable phosphor to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the stimulable phosphor with an electromagnetic wave such as visible light or infrared rays (hereinafter referred to as "stimulating rays") to release the radiation energy stored in the phosphor as light emission (i.e., stimulated emission); photoelectrically detecting the emitted light to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals. The phosphor sheet thus processed is then subjected to a step for erasing a radiation image remaining therein, and then stored for the next radiation image recording and reproducing procedure. Thus, the stimulable phosphor sheet can be repeatedly employed.
In general, a support and a protective film are provided on the top and bottom surfaces of the stimulable phosphor sheet, respectively. The stimulable phosphor sheet usually comprises a binder and stimulable phosphor particles dispersed therein, but it may consist of agglomerated phosphor without binder. The stimulable phosphor sheet containing no binder can be formed by deposition process or firing process. Further, the stimulable phosphor sheet comprising agglomerated phosphor soaked with a polymer is also known.
The radiation image recording and reproducing method is often used as X-ray radiography for medical diagnosis. In that case, it is especially desired to obtain a radiation image of high quality (particularly, high sharpness for high resolution) by applying a small dose of radiation. Therefore, the stimulable phosphor sheet is required to have a high sensitivity and to provide an image of high quality.
The sharpness of radiation image is mainly affected by diffusion or scattering of the stimulating rays in the stimulable phosphor sheet in the case that the reading process is performed by the steps of sequentially scanning the stimulating rays to divide the plane of the phosphor sheet into plural pixels, and detecting the stimulated emission by a photodetector. In this process, the procedure for reading the latent image comprises the steps of sequentially applying a light beam of the stimulating rays onto the surface of the phosphor sheet to induce the stimulated emission, and successively collecting and detecting the emission. If the stimulating rays diffuse or scatter on the place of the phosphor sheet, the stimulating rays excite the stimulable phosphor not only at the target pixel but also in its adjacent pixels. Consequently, the stimulated emission emitted from the target pixel is contaminated with that from the adjacent pixels. The contamination of the emission impairs the sharpness of the reproduced radiographic image.
For avoiding the diffusion or scattering of the stimulating ray in the stimulable phosphor sheet, it has been proposed to divide a plane of the stimulable phosphor sheet into small physically divided sections (microcells) with a partition reflecting or absorbing the stimulating rays.
Japanese Patent Provisional Publication No. 59-202100 discloses a stimulable phosphor sheet having a honey-comb structure consisting of a great number of microcells filled with stimulable phosphor. This phosphor sheet comprises a substrate and a stimulable phosphor layer provided thereon, and the honey-comb structure sectioned with a partition is further provided on the phosphor layer.
Japanese Patent Provisional Publication No. 9-15396 describes a stimulable phosphor sheet which comprises a grid having openings with a size larger than a beam spot of stimulating rays to define an emitting area of one pixel by the grid opening so that the resulting radiation image can have a high space resolution.
According to the study of the present inventor, the stimulable phosphor sheet comprising a grid and plural microcells surrounded by the grid in its stimulable phosphor layer generally has nonuniformity in the sensitivity on the respective microcells. The nonuniformity of sensitivity is caused by nonuniformity of the phosphor layer which is prepared by coating a stimulable phosphor-containing solution on a plane having a grid thereon.
The stimulable phosphor sheet having non-uniform sensitivity gives a reproduced radiation image of low quality because it has a structure noise due to nonuniformity of the sensitive material. Moreover, if the scanning space (space between one scanning line and next scanning line) is not identical to the sizes of microcells, a moire (i.e., a pattern of interfering stripes) is observed on the reproduced radiation image, to reduce the quality of the radiation image.